Lightning-arrester block



l O 6 l CROSS RIzHLRENCE EXAMN Aug. 19, 1941. F. B. JQHNSQN 2,253,376

LIGHTkING-ARRESTER Locx Filed June 25. 1959 myENToR Patented Aug. 19, 1941 2.253.376 LIGHTNING-ARRESTER BLOCK Frederick B. Johnson, Murrysvllle, Pa., assignor to Westinghouse Electric Manufacturing Company, East Pittsburgh. Pa., a corporation of Pennsylvania Application June `23, 1939, Serial No. 280,677

7 Claims. (Cl. 7201-;

My invention relates to high-resistance molded blocks of composition-material, auch as are utilized as excess-voltage protective devices, because of their valve-type characteristic which enables them to readily discharge high currents at excessive voltages and to greatly increase their resistance to a discharge at normal voltages. Such blocks have terminal-ends which are commonly provided with a sprayed-copper coating in order to eect a reasonably good electrical connection, and in general, this sprayed-metal coating cannot be permitted to reach quite to the extreme edges or periphery oi.' the end-surface. because of the necessity for keeping the sprayed copper oi! of the sidewalls of the block.

Such blocks display a very marked tendency to fail along' their sides, and various remedies have been proposed therefor. In accordance with my sectional plane indicated by the une n-n in of silicon-carbigf granules or crystals and a binder. maii't' ze, for the binderl any one of 10 various ceramic or cla binders, or other materials inclua'g v'arious silicaA tesI but at present I prefer to utilize a bin 'er of 4water, glass. which is a commercial, -water-solul le so`du""m silicate, which may be defined as any one or more of" a l5 number of chemical compounds consisting of various proportions of silica, soda, and water.

By the term block I means to refer to any suitably shaped mass of material, the illustrated block being in the form of a short length of a present theories, I attribute such failures to the cylindrical body of material having nat ends' concentration of current at the edges of the sprayed-copper facings, and I explain the failures on the basis of the tendency of such high-resistance composition-materials to hog" the current. that is, to reduce their resistivity when overheated by high current-flow. so that the spot which begins to become overheated by excessive current concentration gets worse and worse, by cumulative action, until a failure is produced.

It is an object of my invention to provide an havin a hi her order of resistivit than th main I improved block in which means are provided 3 E g y e for preventing excessive current-concentrations. around the sides.

-More specifically, it is anobiect of my invention to provide a block which is molded with a non, peripheral layer of insulating composition mav terial, or of material which is at least oi' a higher order of resistivity than the average resistivity of the main body-portion of the block. This materia] merges somewhat. with the main bodyportion of the block, at the junction point, creating a region of tapered resistivity, and it also provides an insulating, or extremely high-resistance, marginal space, at the ends, in which the sprayed-copper coating may terminate without creating a region oi' high current-density at the edge of the sprayed-copper coating.

With the foregoing and other objects in view,

`.myinvention consists in the structures, compositions and methods hereinafter described and claimed, and illustrated in connection with the accompanying drawing. wherein:

Figure 1 is a plan view of a block made in accordance with my present invention;

although I wish it to be understood that I am not limited to ans7 particular length or shape.

In accordance with my present invention, the block is formed, preferably in the original mold- 25 ing process, with a small annular edge-portion 2 consisting of a molded composition-material also utilizing a water glass binder. but with a granular ller-material which is substantially an insulator, or which at least results in a product body-portion I. The filler-material which is utilized in the peripheral portion i may be finely subdivided felgsgar sand. any one of various metallic oxi es, ora ost any other nely divided Y ng material, insulating or non-insulating, even including sili crystals of a different size or quantity than the crystals which are utilized in the main body-portion l.

In the process of manufacture, various methods 40 are available for so molding the block that it has the main body-portion i and the marginal por. tion 2 as described, after which the whole body. consisting of the portions I and 2, is dried out,

by baking t0 Prxlgwuelndeene- Ixfpreferredp" marginal portion 2 is portions I and 2 readily merge together, providing an intermediate-region 3 in which the highresistance property of the main central block l tapers oil toward the insulating, or at any rate still higher-resistance. properties of the marginal Fig. 24 is a cross-sectional view thereof. on a 55 rim 2t In this manner, I avoid sharp concentrations oi' current along or near the edges of the main body-portion I.

After the molding and baking operations, the block is provided with terminal conductive coatings l and l of sprayed copper or other good conducting material, preferably metallic, and this sprayed-copper coating preferably terminates. at its peripheral edge, in the flat end-portions of the marginal rim-portions 2 of the molded block, so as to avoid the peripheral current-concentrations which are obtained when the marginal edge of the copper-spray terminates on the nat endsurface of the main body-portion I.

In l'iig- 3, I show a modied form of embodiment of my invention, in which the marginal cylindrical-shaped annular molded portion 2 is replaced by two conical portions 6 and 1 which encircle the main body-portion I only at the flat ends which are coated with the sprayed-copper coatings l and l, so that the main body-portion I has a greater diameter at its central portion than at its ends. I'his necessarily -results in som'e concentration of current around the edges of the main body-portion I, at the reduced-area ends, where the diameter of the main body-portion I is reduced by reason oi the presence of the conical peripheral portions 8 and 1, but the blended junction-portions l, where the material 6 and 'I `loins with the main body-portion I, serves somewhat in the capacity of providing a peripheral region 9 of gradually increasing resistivity, in the ends of the block, so`as to spread out the current-concentration over a wider area. thus reducing its intensity.

My invention, particularly in the preferred form shown in Figs. 1 and 2, has resulted in blocks of improved performance, and by making the cylindrical marginal portion 2 with a illler material which is less abrasive than the silicon carbide of the main body-portion I, I secure a saving in the wear and tear on the molds which are needed in the manufacture oi' the block.

I claim as my invention:

l. An excess-voltage protective device comprising a nat-ended high-resistance block comprising a main portion oi high-resistance molded composition-material terminating in said il'at ends and having a valve-type characteristic which enables it to readily discharge high currents at excessive voltages and to greatly increase its resistance to a discharge at normal voltages, a closely adhering edge-portion extending around at least each end oi said main portion and also terminating in said fiat ends, said edge-portion being of a higher order of resistivity than the average resistivity oi' said main portion, and a coating of agood conducting material covering each of the ilat ends, each goodconducting coating covering all of said main portion and at least some of the surrounding edgeportion at its end of the block.

2. An excess-voltage protective device comprising a nat-ended high-resistance block comprising a main portion 'oi' high-resistance molded composition-material terminating in said nat ends and having a valve-type characteristic which enables it to readilyI discharge high currents at excessive voltages and to greatly increase its resistance to a discharge at normal voltages, a

closely adhering edge-portion extending around, and entirely covering, the sides of said main portion, said edge-portion also terminating in said ilat ends, said edge-portion being of a higher order of resistivity than the average resistivity of said main portion, and a coating of a good conducting material covering each of the flat ends, each good-conducting coating covering all of said main portion and at least some ot the surround- Ing edge-portion at its end of the block.

3. An excess-voltage protective device comprising a flat-ended molded high-resistance block ,adapted to carry current from end to end thereof and comprising two different mixtures, each composed of finely subdivided material and a binder, the main portion of the block being of a mixture resulting in a high-resistance body-portion terminating in said ends and having a valvetype characteristic which enables it to readily discharge high currents at excessive voltages and to greatly increase its resistance to a discharge at normal voltages, the sides of said main portion being surrounded, and entirely covered, by material of a different mixture resulting in a surrounding-portion of a higher order of resistivity than the average resistivity of said main portion, a common binder being utilized for both mixtures, and the whole being molded together in an adhering integral mass, and a coating of a good conducting material covering each of the ilat ends, each good-conducting coating covering all of said main portion and at least some of the surrounding-portion at its end of the block.

4. An excess-voltage protective device comprising a fiat-ended molded high-resistance block adapted to carry current from end to end thereof and comprising two dinerent mixtures, each composed of `vilnely subdivided material and a binder, the main portion of the block being of a mixture resulting in a high-resistance bodyportion terminating in said ends and having a valve-type characteristic which enables it to readily discharge high currents at excessive voltages and to greatly increase its resistance to a discharge at normal voltages, the sides of said main portion being surrounded, and entirely covered, by material of a different mixture resulting in a surrounding-portion of a higher order of resistivity than the average resistivity of said main portion, a common binder being utilized for both mixtures, and the whole being molded together in an adhering integral mass, and a coating of a good conducting material covering each of the ilat ends, each good-conducting coating covering all of said main portion and, at least some of the surrounding-portion at its end of the block.

5. The invention as defined in claim 2, characterized by said edge-portion comprising a molded mixture including a nnely subdivided nller and a binder comprising a water-soluble silicate.

6. I'he invention as defined in claim 3, characterized by the common binder being an aqueous mixture including water-soluble silicate.

7. The invention as denned in claim 4, characterized by the common binder being an aqueous mixture including water-soluble silicate.

FREDERICK B. JOHNSON. 

